<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var a = 'abc';   //转换成数字再进行计算
    console.log(parseInt(a)); //NaN Not a Number  isNaN()
    var b = 20;
    c = a - b;
    if (isNaN(c)){
        alert('提醒')
    }
    console.log(c);

    //+没有进行数据类型转换，+还可以连接字符串
</script>
</body>
</html>